The present invention relates to a slippage control method and device for a four wheel drive power transmission system for a vehicle, and more particularly relates to such a slippage control method and device for such a four wheel drive power transmission system for a vehicle such as an automobile adapted for four wheel drive operation, particularly adapted to control the differential action of a differential device which is provided for distributing power between the front wheels of the vehicle and the rear wheels of the vehicle, in which the construction and operation thereof are improved so as to improve the quality of slippage control and thereby improve vehicle drivability and other operational characteristics.
The present invention has been described in Japanese Patent Application Ser. Nos. Showa 60-280662 (1985), Showa (61-061801 (1986), Showa 61-061802 (1986), Showa 61-065314 (1986), Showa 61-089347 (1986), Showa 61-105468 (1986), Showa 61-105542 (1986), Showa 61-125197 (1986), Showa 61-149079 (1986), and SHowa (61-161298 (1986), all of which were filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the text of said Japanese Patent Applications and the claims and the drawings thereof; copies are appended to the present application.
Nowadays a greatly increasing number of automotive vehicles are being constructed with four wheel drive transmission systems, because such four wheel drive operation, in which all four wheels of the vehicle are powered from its engine via its transmission, is very suitable for driving on poor or slippery road surfaces such as in mud or over bad ground, or upon roads covered with mud, snow, ice, or rain. In other words, four wheel drive operation provides a much higher degree of stability and drivability for the vehicle in conditions where the coefficient of friction between the wheels and the surface upon which the vehicle is riding is relatively low. Also, four wheel drive operation is beneficial for aiding with hill climbing characteristics and high speed stability characteristics. Therefore, the so called full time four wheel drive type of transmission, which remains always engaged to four wheel drive without any episodes of two wheel driving, is becoming more and more popular.
In such a four wheel drive transmission system for an automotive vehicle, it is usual to provide a center differential device for distributing rotational power between the front wheels of the vehicle and the rear wheels of the vehicle, as well as the per se conventional rear differential device that provides differential action between the two rear vehicle wheels and the also per se conventional front differential device that provides differential action between the two front vehicle wheels. Such a central or front-rear differential device is provided in order to provide a differential action between said front vehicle wheels (considered as a pair) and said rear vehicle wheels (also considered as a pair) when the vehicle is turning around a curve, in order to eliminate the possibility of the occurrence of the so called tight corner braking phenomenon created by the difference in the turning radiuses of the front wheels of the vehicle and the rear wheels thereof. And such provision of such a central or front-rear differential device is effective for achieving this result. However, a problem that arises with such provision of such a central or front-rear differential device is that, if at any time even one only of the four vehicle wheels should break away from the road surface and should start to spin, then no drive power or at least severely reduced drive power is provided to the other three vehicle wheels. This type of problem is particularly likely to arise in the event that the road conditions are poor due to rain, snow, dust, dirt, or the like which deteriorate the coefficient of the vehicle wheels on the road surface, and thereby vehicle drivability can be severely reduced.
In order to counteract this effect, it has been practiced to provide a device to such a front-rear differential device which prevents said front-rear differential device from performing differential action, in a selective fashion. When such a center differential action inhibition means, which typically may be either a viscous fluid type friction coupling or may be a friction engaging means such as a hydraulic clutch or a hydraulic brake, is actuated, it causes the differential action provided by said front-rear differential device between the front vehicle wheels and the rear vehicle wheels to be at least partially prevented, and instead said front vehicle wheels, considered as a pair, are driven from the vehicle engine, and also said rear vehicle wheels, considered as a pair, are at least partially independently driven from said vehicle engine. Thereby, the problem outlined above, of loss of power to the other three vehicle wheels when one of the vehicle wheels starts to spin, is at least partially obviated. Such types of structure are at least partly disclosed, for example, in Japanese Utility Model Application Laying Open Publication Ser. No. 47-203 (1972), Japanese Patent Application Laying Open Publication Ser. No. 50-147027 (1975), Japanese Patent Application Laying Open Publication Ser. No. 55-72420 (1980), Japanese Patent Application Laying Open Publication Ser. No. 57-15019 (1982), Japanese Patent Application Laying Open Publication Ser. No. 58-20521 (1983), Japanese Patent Application Laying Open Publication Ser. No. 58-20521 (1983), Japanese Patent Application Laying Open Publication Ser. No. 59-151661 (1984), Japanese Patent Application Laying Open Publication Ser. No. 59-184025 (1984), Japanese Patent Application Laying Open Publication Ser. No. 59-206228 (1984), and Japanese Patent Application Laying Open Publication Ser. No. 60-176827 (1985), none of which is it intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law. Also, attention should be paid to the not yet published Japanese Patent Application Ser. Nos. 60-280662 (1985), 61-65314 (1986), 61-87655 (1986), 61-105474 (1986), 61-105475 (1986), and 61-105542 (1986), again none of which is it intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law.
Further, in the event that the front-rear differential device is of an unequal distribution type which distributes drive torque substantially unequally between the front vehicle wheels and the rear vehicle wheels, then, during periods in which said front-rear differential device is not being prohibited from providing its differential action by the above mentioned differential action inhibition means, the amounts of torque distributed between the front vehicle wheels and the rear vehicle wheels are different. In the case that the amount of torque distributed to the rear vehicle wheels is larger than the amount of torque distributed to the front vehicle wheels, the performance of the vehicle for starting off from rest is improved; while, in the converse case that the amount of torque distributed to the front vehicle wheels is larger than the amount of torque distributed to the rear vehicle wheels, the performance of the vehicle for straight ahead driving operation, and the stability of such straight ahead driving operation, are improved.
The following type of problem, however, can tend to arise in a so called full time four wheel drive vehicle fitted with such a front-rear differential device equipped with such a differential action inhibition means, in the particular case that said differential action inhibition means is a viscous fluid type friction coupling. Such a viscous fluid type friction coupling provides a drag effect which increases as the difference between the rotational speed of its rotational power input member and the rotational speed of its rotational power output member increases, and according to this action it is impossible, if the differential action inhibition means is a viscous fluid type friction coupling, for the differential action between the front wheels of the vehicle and the rear wheels of the vehicle to be totally locked up or prevented. In other words, the greater the differential rotation speed of the front vehicle wheels and the rear vehicle wheels, the greater the torque transmission capacity of the differential action inhibition means, but the front vehicle wheels and the rear vehicle wheels cannot ever be completely connected together. With increase in its capacity, such a differential action inhibition means which is a viscous fluid type friction coupling can produce an operational state close to the locked up state, but along with this increase in capacity, even when the differential rotation speed of the front vehicle wheels and the rear vehicle wheels is relatively low, the torque transmission capacity is relatively large, and therefore the differential restriction effect provided is excessive. On the other hand, if a relatively small capacity such differential action inhibition means is utilized for the viscous fluid type friction coupling, certainly when the vehicle is proceeding around a curve or corner the tight corner braking phenomenon will be prevented from occurring, but the tire slippage amount will be considerably increased, and unless the differential rotation speed of the front vehicle wheels and the rear vehicle wheels is increased during tire slippage the necessary differential restriction rate will not be obtained, and although the loss of driving power on all the vehicle wheels will actually be prevented nevertheless an increase in the driving force exerted on the tire or tires which has slipped cannot be expected. Moreover, in the case where such a viscous fluid type friction coupling is fitted as such a differential action inhibition means to the front-rear differential device, the torque transmission capacity thereof will inevitably depend strongly upon the temperature of the working fluid thereof, and it is difficult to maintain the selection of an appropriate torque transmission capacity under all conditions of use.
On the other hand, in the case that a friction engaging mechanism such as a hydraulic clutch or a hydraulic brake is fitted as such a differential action inhibition to the front-rear differential device, the typical action thereof is an ON/OFF action with complete engagement and complete disengagement, but with such switchover the manner of four wheel driving inevitably changes suddenly, and, particularly in the circumstances of a manual transmission operational regime, the decision as to when to perform such ON/OFF action for the friction engaging mechanism is difficult, and in practice appropriate configuration and operation are very difficult.
Further it has been proposed, in order to avoid the occurrence of the tight corner braking phenomenon, to release the differential action inhibition means when the vehicle is going around a corner; but it is very difficult in practice to carry out proper control of said differential action inhibition means so as to be satisfactory in all vehicle operational conditions, since driving conditions for the vehicle may rapidly fluctuate. For example, if the differential action inhibition means is always released when the vehicle is going around a corner, when the coefficient of friction between the tires on the vehicle wheels and the road surface is relatively large, the occurrence of the tight corner braking phenomenon is indeed substantially prevented, but, in the contrary case when the coefficient of friction between the tires on the vehicle wheels and the road surface is relatively small, slippage of the tires on the road surface tends to occur, and effective vehicle driving force may no longer be obtained. To carry out optimal control it is in theory preferable to detect the actual ongoing value of said coefficient of friction between the tires on the vehicle wheels and the road surface, and to carry out the control of the differential action inhibition means in the light thereof, but in practice it is difficult to detect this coefficient of friction. However, said coefficient of friction between the tires on the vehicle wheels and the road surface is the most important factor governing whether slippage of the vehicle wheels is likely to occur or not, and said coefficient of friction can vary over an extremely wide range, depending upon whether or not rain or snow is falling, and upon whether or not the road surface is covered with a film of frozen frost or with ice, among other factors.
Further, various concepts have been mulled for controlling the engagement and the release of the differential action inhibition means according to the vehicle operational conditions, but one problem has been that when said differential action inhibition means is changed over from the released condition to the engaged condition a certain delay occurs in its action, and further when said differential action inhibition means actually starts to be engaged a sudden rise in torque transmission provided thereby occurs, which can cause a large transmission shock.
The following problem also can arise with such a system. Generally, in a vehicle such as an automobile, the front wheels thereof are the wheels which provide steering action for the vehicle, and therefore, particularly if slippage should occur between the front wheels and the road surface, the quality of vehicle steering performance will be severely compromised, and vehicle stability will be deteriorated.
Further, the above outlined problems are particularly troublesome in the case that the transmission of the vehicle is of the continuously variable type: this presents some specific problems of implementation with regard to avoidance of the tight corner braking phenomenon.
Now, the previously mentioned Japanese Patent Application Laying Open Publication Serial No. 55-72420 (1980) discloses a four wheel drive device constructed so that, during four wheel drive operation, when the difference between the rate of rotation of the rear vehicle wheels and the rate of rotation of the front vehicle wheels is at least a certain value, in other words when slippage is occurring between at least one vehicle wheel tire and the road surface, the differential control clutch is engaged and the front vehicle wheels and the rear vehicle wheels are directly coupled, while at other times during four wheel drive said center differential control clutch is disengaged and the center differential device is allowed to carry out its differential function. But if the question is asked as to whether or not it is satisfactory that when slippage occurs between at least one vehicle wheel tire and the road surface the front vehicle wheels and the rear vehicle wheels always and uniformly should be completely directly coupled together, the answer is that, on the contrary, if the rear vehicle wheels and the front vehicle wheels are completely directly coupled together by said differential control clutch, then there is a danger that with variations in the coefficient of friction of the road surface being driven upon slippage will occur between the rear vehicle wheels and the front vehicle wheels together and the road surface.
In the particular case that said differential control clutch is a wet clutch, the viscosity of the lubricating oil therefor inevitably changes with temperature, and as a result the torque transmission capacity thereof fluctuates with such changes in the temperature of its lubricating oil; even for the same clutch engagement pressure, the torque transmission capacity is lower if the lubricating oil temperature is higher and on the other hand is greater if the lubricating oil temperature is lower.
Therefore, in the summer the torque transmission capacity of such a wet clutch will tend to be insufficient, whereas in the winter there is a tendency for the torque transmission capacity of such a wet clutch to be excessive.
In particular, when the lubricating oil of the wet clutch is one and the same as the operating oil of the vehicle automatic speed change device, then the temperature thereof will be influenced by the operation of the fluid torque converter of the vehicle automatic speed change device, and it is a fact that usually there are relatively large changes in the temperature of such lubricating oil, and therefore particularly in this case the fluctuation in the torque transmission capacity of the wet clutch is large, and there is a danger that the desired control of the torque transmission capacity will not be possible.